These Fuses Melt in Water

Back in 80s I was a calibration technician in the US Air Force, working in an Electronic Warfare shop. We had a major expansion that doubled the size of the facility. After the normal new building problems (leaky roof, phones not working, etc.), we had a new, shiny building with plenty of workspace, storage space, a break room, and even a shower.

After about six months, however, some strange electrical problems started showing up. Because this was Florida during the summer, we normally had to shut down for afternoon thunderstorms and lightening advisories to protect the tens of millions of dollars of test stations, avionics, and other systems. It took a while for the patterns to emerge from the normal glitch noise level.

First, my test station started blowing 45A fuses in the disconnect box mounted on the wall. This wasn’t too uncommon, but one of the station power supplies would usually go first. We would replace the fuse and bring the station back up, stage-by-stage, until we found the fault. Sometimes the fuse would just blow -- or so it seemed. The pattern started to emerge when a fuse blew about once a week. We would troubleshoot each time, but couldn't seem to find the problem.

The frequency increased until we were blowing a fuse a day. We then realized that we were only blowing the B phase fuse. At this point, we called in the electrician from base Civil Engineering. His clamp-on ammeter indicated we were only drawing about 17 amps per phase and the phases were fairly well balanced. The CE electrician had run out of 4517A fuses and only had 60A fuses in his kit, so he installed one.

After some lively discussion, we noted the station was on a 50A circuit, and we needed to get back up and running while the electrician ran across base to get the correct fuse. In the meantime, the 60A fuse blew and the 50A breaker didn’t. This time, instead of sending it straight to the trash, I picked up the fuse from the fuse puller to check the rating and burned my fingers.

That was the aha moment. We weren’t blowing the fuses, we were melting them! After shutting the main breaker off, I started probing and prodding the disconnect box, and I found some discoloration on one fuse holder clip and the terminal lug underneath it. The ohmmeter indicated more than 1 ohm on this heavy gauge connection. P= I²R. The 17A draw was enough to cook the fuse until it melted. A closer look revealed water marks inside the disconnect box. The box was mounted on what was the outside wall before the expansion.

The new roof had overlapped the old one, resulting in numerous leaks when we moved in. Water had gotten into the box and corroded the contact. The leaks had been fixed and the damage done before we even moved in.

The electrician returned about an hour later with the new fuses. I explained my findings and indicated that the box needed to be replaced, even after cleaning up the contacts. After three more electricians were called out to view the problem, the senior CE electrician finally agreed and the box was replaced about a week later.

the link at the bottom of mr bandit's post can be inspirational to those folks who don't want to be told what they cannot do. Adm Grace Murray Hopper is one of my heroes (heroine?). Just to add a bit to the linked info, Adm Hopper was interviewed on 60 minutes, was a regular on the lecture circuits (no pun intended) and is the namesake of the Computing center building at the Naval Air Station at North Island (San Diego), CA in addition to the (Navy) combatant ship named after her.

I think she serves as a great example to anyone considering a career in technologies or computers, but especially for young ladies and girls who wonder if they can become leaders in high tech areas (may not be so much doubt about that now), but it was the likes of Amazing Grace who led the way 65 years ago.

Never had a *bug* in my AC unit, but I did have a relay in the outdoor compressor unit fail several times. The relay was an unsealed, open type, which was located near the bottom of the unit. Since the housing of the unit was open at the bottom, the relay was (mostly) protected from rain, but any time we had to mow the grass or weed-whack the growth around the compressor, clippings would be driven with enough force to "hook" up under the lip of the housing and hit the relay, carrying enough moisture to eventually cause corrosion. Ditto for snow during the winter. The first time that relay failed, it jammed *closed*, and kept the compressor running constantly for *weeks*. We didn't notice b/c the compressor was a quiet type, and the central air blower inside the house only activated when the thermostat ordered it (and I was working very long hours). I first noticed something odd when i had to enter the crawlspace and found the high-pressure coolant line was dripping condensation over its entire length constantly, even when the central air wasn't running. Ended up tracking down the culprit by killing the thermostat power circuit and realizing that the compressor kept running. The relay was only $20, and I replaced it myself easily enough, but good grief, the electric bill for that month was ENORMOUS.

That relay became the "first check" item whenever there was an AC problem for the next several years. It never jammed closed again, but it did fail a few more times. Why anyone would use a relay with mechanical components open to the elements in an installation like that is beyond me.

Actually, the term "bug" came from telephony, as in "sounds like bugs on the wire". I suspect early telephone equipment were also invaded by bugs, but that is just a guess. Steven Levy's book Hackers goes into this a bit, IIRC.

She found the moth in a relay (relay #70, panel F), and taped it into her log book with the note "First actual case of bug being found". The logbook page is in the Smithsonian.

google "Grace Hopper bug" for pictures of the page (92) and the moth. Date was 9/9, 1947. One good source:

I am not an electrical engineer, but I think I know how fuses work. Can you explain why any fuse would cost $350? Even if they are only produced in small quantities that sound outrageous. I have quoted supplying components for a large supplier of fuses who is headquartered in this area and I have never been successful. They throw pennies around like sewer lids.

There's NO question about the legitimacy of including extra (unenergized) circuit wires in a raceway. However, in THIS case, this is NOT the situation. I believe the problem may be traced back to the condition that I described above. When I peered into the 2-gange switch box on the wall, there is only one cable w/ a 3-wire configuration, and that cable goes to a receptacle on the wall. One part of that duplex receptacle is live all the time, the other is connected to the red wire, which allows the convenience to be able to switch a table lamp or bedside lamp as one enters / leaves the room. The other switch in the box is wired to a cable that goes to the ceiling box for a light and/or fan device. This cable has NO red wire showing at the switchbox terminus, BUT there IS an unenergized red wire in the ceiling box. I will look more closely to determine IF the other end of this red wire has been cut as it enters the switch box. Wouldn't surprise me one bit. I've seen some very weird and unacceptable constructs in my lengthy career.

An inspector WOULD NOT take notice of a minute detail such as this, and once the house was "walled in", it would be too late to rectify this problem. Unfortunately, in this case, the house was rented for about 4 years from the time it was first built. And, with a 2-year empty status due to a foreclosure, the problem was not discovered until now, and now it's too late to seek any remediation from the original parties involved.

The "myth" about bugs in a computer is NOT a myth. But, that is where the term originated. When the first ENIAC was fired up about 1948, there were thousands of relays used as memory cells. A moth had gotten itself swept up into the guts of the computer, causing one of the relays to not make when it was supposed to. After much troubleshooting, the moth was discovered & removed. The ENIAC continued to calculate. Dr. Hopper of the U.S. Navy was part of that event in history.

I've used the $15 ones with great success. In fact, when they were on sale for $10, I bought a few for my sons and friends. The major difference is that they don't have the targeting laser--you actually have to pay attention to where you aim them.

I used one of them to determine why CFL lamps were failing after only a few months in my ceiling fan wihich has they typical base-up mounts with small shades.

30 seconds after turning the lamp on from a day at 70-degree room temperature the lamp base was at 155 degrees F. Question answered.

I am pretty sure that having extra ("spare") un-energized conductors present in home wiring is not a code violation, and wouldn't prevent a wiring job from passing inspection. Perhaps our colleagues who are more knowledgeable regarding the NEC can comment on this. I do agree, however, that poor workmanship (and poor design) is prevalent in home wiring. If you want something done right, do it yourself!

This summer I added a DCU to an AC Compressor outside. The DCU has conservation cycles to prevent brownout for electric grid. The DCU has no ok status led only a running status led when it is doing a cycle. The inside of house started to get hot one day and by second day I noticed that I should check the system. So, I lowered the thermostat to make a call to air handler. The thermostat status would show running so I though the issue wasn't the thermostat. Next, I checked the air handler and noticed that during the thermostat run cycle was enabled that the air handler would run. So, I though, okay it's not the thermostat or air handler and the issue had to be outside and the DCU was causing the request from air handler to be denied to compressor because there was no okay status light running or test or reset switches.

So, I called the electric company and a technician came out and checked the DCU. We doubled checked the breakers, ohm checked fuses to compressor and the electric to compressor. Everything was checking out a-okay. Now there was only one more thing to check which was the DCU low voltage signals to a relay switch for the high voltage and noticed that the relay wasn't making the contact.

So, the technician looked real hard with flashlight at the contacts of relay and noticed something was between the relay contacts. It was a little bug about half inch long stuck between the relay contacts. Throughout the years I have noticed a whole lot of issues that arise out of relays. However, that little bug knocked the whole system down. It reminds me of the myth from IBM's first relay computer story about bugs in the system. This is where the term bugs in your system comes from.

One time I couldn't access my workstation with username and password using the enter key on keyboard. So I tried a whole lot of times of accessing and than onetime instead of using keyboard enter key I used the mouse and clicked the ok button and than I was able to login. So I logged out and tried the keyboard enter key and noticed that when I pressed the enter key another character was being added to the password when I pressed the enter key. So tried the mouse again and I was able to login. Logout and tried the keyboard enter key and again the added character was added to password.

So, I did a process of elimination of sequences and used the rule number one, always check you're connection first. So, I unplugged the keyboard cable and looked at it real good and noticed the cable had a small little nick in the cable. Than I looked, how the nick was caused and what was happening to the cable during normal usage. What I discovered was, the keyboard sat on a keyboard-sliding tray and when the keyboard tray was pushed in the cable would get caught between the sliding part and this must of cause the nick in the cable.

But the darness thing about the whole thing was the extra character that was added to the password when I would press the enter key and back tracing my sequences.

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